Photolithography is a process by which integrated circuits are manufactured in the semiconductor industry. A photomask is an opaque plate with holes or transparencies that allow light to shine through in a defined pattern.
Lithographic photomasks are typically transparent fused silica blanks covered with a pattern defined with a chrome metal absorbing film. Photomasks are used at wavelengths of 365 nm, 248 nm, and 193 nm. This is shown as the light source 101 in FIG. 1, which then propagates through the exposure slit 102. Photomasks have also been developed for other forms of radiation such as 157 nm, 13.5 nm (EUV), X-ray and electrons and ions. As shown in FIG. 1, a set of photomasks 103, each defining a pattern layer in integrated circuit fabrication, is fed into a photolithography stepper or scanner and individually selected for exposure. Light passing through the photomask goes through the reduction optics 104. The outcome is a reduced pattern on the silicon wafer 105. Thus a photomask for an integrated circuit is like a negative for a photograph, which must be perfect.
Typically a high end photomask contains several defects. These defects may occur due to problems with the Mask writers or the photo chemistry used to etch the pattern. A photomask is typically written using an electron beam writer or a laser writer. Fluctuations in the beam current or laser voltage may induce non-uniform exposure in the photo resist. This may lead to unwanted discrepancies in the pattern. Alternatively, defects may be induced by the photo resist itself or even imperfection in the blank mask substrate. For EUV masks, defects in the blank substrate are very common, thus any pattern written on it will also be defective. A defect or unintended pattern on the photomask will propagate to all wafers exposed using it.
Photomask inspection or reticle inspection is an operation of checking the correctness of the fabricated photomasks to ensure that there are no defects in the pattern. This process is typically performed using a mask inspection system. Upon completion, the inspection system generates an inspection report (IR). As shown in FIG. 2, this IR report contains the sequential defect number, the X and Y coordinates of the defect with respect to known frame of reference 201, and sometimes an image of the defect and reference patterns. Typically the operator visually looks at every defect and assigns it a defect classification code that later aids in the mask making process. Since the wafer pattern is a replica of the photomask printed at a fixed reduction level, the wafer may also be used to check for defects found on the reticle. The bottom left corner of the mask is identified by 202. The inspection report may also contain coordinates of the alignment points 203 and 204 used to align the physical mask with the database reference pattern.
There are generally two modes in which the inspection systems operates. In the first mode, as shown in FIG. 3, the pattern on the mask, sometimes called a die, is compared against an ideal database reference pattern. This is generally referred to as a die-to-database inspection. In the die-to-database inspection the location of the defect 301 is precisely known.
In the second mode, a photomask having multiple repeated dies that can be compared to each other to look for defects. This mode of operation is called a die-to-die inspection, for which a database reference pattern is not required. If the dies are only repeated once, then comparing the left die with the right die, only concludes that there is a discrepancy between the two dies. This does nothing to indicate which of the two dies is indeed defective. As shown in FIG. 4, defect 401 in the left die and defect 402 in the right die are the same X and Y coordinate in each of the two identical dies. Using this method of error detection, the inspection tool will assume the left die is accurate and to be used as a reference and as such report the defect coordinate in the right die. This method has the limitation that the coordinate reported may be of the reference die. Therefore, the defect coordinate 402 must be translated by the die pitch distance 403 to accurately result in the defect location as represented by 401.
This problem however does not exist when the same die is repeated two or more times in the scanning direction, whereby through a process of elimination the exact die that contains the defect is automatically reported by the inspection tool.
A defect location is always reported, by its X & Y coordinate, with respect to a fixed reference point on the mask. This reference point may not be the same physical location and varies based on the type of inspection, the brand of the inspection tool, or even the choice of the operator. For patterned mask inspection the defect origin is generally defined by a special reference point feature 201 on the mask. When the inspection is started the stage of the inspection system is aligned to this reference point. For this reason the location of the defect is very precisely known with respect to the reference point.
Not all inspection systems use the same reference point as the origin. FIG. 5 shows a defect map where the reference point used is on the top left corner of the mask 501. Even though the defect map in FIG. 5 is identical to that shown in FIG. 3, the coordinates of each defect are much different because they have different origins (reference points).
Masks may be loaded into the inspection tool in various orientations. For example it may be that when the mask is covered with a pellicle, a transparent framed film to prevent micro contaminations from falling on the mask pattern, it may not be possible to load the mask in the standard orientation into the inspection system's stage. Other times, a given model of the inspection system may expect the mask to be loaded in a rotated orientation onto the stage. Regardless of the reason for the orientation change, the defect map looks rotated as shown by 601 in FIG. 6, when compared to FIG. 3 or FIG. 5. The orientation change induces a further level of complexity when matching defect locations across multiple inspections of the same mask. Not all mask inspections are done for the patterned masks. For advanced masks, it is important that the blank mask be inspected for defects before the pattern is etched on it. This is necessary to ensure that the blank mask does not have large defects on which it may not be possible to write a feature of the pattern. Since a blank mask does not have a reference point, the defect coordinates are given with respect to the bottom left corner of the inspection tool or some other coordinate frame origin such as the mask center. Since the bottom corner of the mask is not very well defined, or even its center, the location of the defect is not very precisely known. This is because there is no reference mark on the mask yet, to which the stage can be aligned (FIG. 7). To align the mask, an operator simply places the bottom corner 701 of the mask against the bottom corner of the stage of the inspection tool. This is an imprecise method of alignment since the mask can be offset either by particles trapped in-between the stage and the mask or due to other imperfections such as a physical gap. Thus finding a defect that is only a few tens of nanometers on a 154 mm by 154 mm mask, given its X & Y coordinates with respect to a poorly defined origin maybe difficult.
Multiple inspections of the same mask may be needed in the mask making process. For example, the mask may be inspected after the pattern is etched on it, and then any defects needing repair may be repaired, while others may be ignored due to being sub spec. After repair, and prior to shipping the mask, the mask undergoes a final inspection. During this time a repair site may be reflagged by the inspection system, even though upon manual measurement of the defect it may be sub spec. Thus it becomes necessary to track all repeated defects with the ability to match defects in one inspection against all prior inspections using the coordinates of the defects.
These repeated inspections ensure that a prior defect is properly dispositioned. Since there are multiple modes and tools used in inspecting a mask, the process of comparing defect locations becomes increasingly complex. For example, a mask loaded in one orientation during its first inspection may have been loaded in a different orientation in a subsequent inspection. Alternatively, a mask having a reference point near the bottom left corner during its first inspection may have a reference point near the top left corner in a subsequent inspection. Other times it may be necessary to compare defect coordinates on the blank mask against those detected after the pattern is written on the mask. If the defect locations match, than one can attribute the defects on the pattern mask to be due to a defect on the underlying mask blank substrate.
When the mask is shipped to the wafer fab, it too needs to be inspected on a routine basis. This is because one needs to know if new defects are appearing on the mask, that were not there before. Note that inspection systems generally detect many sub spec defects, which the operator manually classifies as being false. Thus, nearly all inspection reports contain defects, some of which are then classified as being false or sub specification. If a defect falls outside the specification, than the mask is sent back to the mask fab for repair.
It is to be noted that the various brands of inspection systems do not all generate an inspection report in a common output file format. For example, some tools write the inspection report as a plane ASCII text file, others may write a XML file, yet others may generate binary output files. Given that a semiconductor fabrication facility may inspect hundreds of photomasks in a given day, this alone presents a huge data management problem.
In order to perform the defect overlay analysis, a user would be required to individually load all inspection reports belonging to a given photomask manually. This alone would prove to be extremely cumbersome as a prerequisite to this exercise would involve saving inspection reports belong to the same mask either in the same file directory or naming all related inspections with a unique identifier as part of the file name, so that they can be found.
Furthermore, every time a new brand of inspection tool is introduced into the process, the defect overlay analysis software will need to be upgraded to be able to understand inspection reports from the new tool.
The alternative would have required multiple individual databases for each distinct inspection tool output format, which would then need to be individually queried to establish a relation between the various inspections of the same mask.
A method is needed that can relate all inspection reports from the various inspection system belonging to the same photomask efficiently.
For such repeated inspections it is highly desirable to compare and track defects across all prior inspection reports. However no current or prior art solution exists that can overlay defects across multiple inspections from various makes and models of inspection systems, having different reference points and orientation. Due to a lack of a viable solution, the knowledge gained by classifying defects in a prior inspection cannot be reused in a subsequent inspection, thereby creating an inefficient process requiring the operator to start from the beginning each time. Sometimes it may not be the mask that is inspected to look for repeated defects. Since the wafer pattern is an exact replica of the pattern on the mask, with a fixed optical reduction factor, inspection reports from the wafer inspection tool may also be used.
The engineers and technicians at a wafer fab generally like to match repeated defects against all previous inspections that were performed on a given mask or wafer, including those that were performed when the mask was being fabricated in the mask making facility. Thus there needs to be a way that makes accessible the multiple inspection performed at the mask fab to the operators at the wafer fabs.
Given that a mask can be inspected in the upright, rotated or inverted orientations, coupled with the problem of inspection systems reporting defects coordinates with respect to different reference points (i.e. origin of the reference frame), presents a huge challenge to the operator. Furthermore an advanced mask fab or a wafer fab can inspect hundreds of mask every day with each inspection containing up to tens of thousands of defects, presents a huge data management challenge on its own. Sifting through the various inspection reports to find the correct ones to compare, alone can take hours of the operators time. Dealing with the various reference frames and inspected orientations, and the intricacies of the various inspection system's output file format further makes this problem extremely difficult to deal with, for which no robust solution exists to date. Therefore, there is a need for a comprehensive solution that addresses the multitude of problems and inefficiencies described above in order to track defects across multiple inspections of the same mask.